49 Thermal Overload Protection
Overview of Function
Thermal overload protection is used to estimate the real-time thermal state (θ) of static plant e.g. cables or
transformers.
Structure of the Function
The thermal overload function has group dependent settings. These settings can be different in each settings
group – group numbers (Gn) 1 to 4.
This function corresponds to the logical node
A49PTTR*
in IEC 61850.
The function monitors the primary system current using the 3 phase CT inputs e.g. CT1/2/3. The RMS current is
measured from processing of the current samples. An output is given for any phase operation.
A calculation of the Thermal State (θ) from the measured currents is based on both past and present current
levels. θ = 0 % for unheated equipment, and θ = 100 % for maximum thermal withstand of equipment or the
trip threshold.
[dw_7SR5_ThermalOverloadHeatingandCoolingCharacteristic, 1, en_US]
Figure 5-12
Thermal Overload Heating and Cooling Characteristic
For given current level, the Thermal State will ramp up over time until Thermal Equilibrium is reached when
Heating Effects of Current = Thermal Losses. The heating/cooling curve characteristic is dependent upon the
Thermal Time Constant, this must be representative of the thermal characteristics for the protected plant.
Similarly the overload setting I
θ
is related to the thermal withstand of the plant.
An alarm is provided for θ at or above a set % of capacity to indicate that a potential trip condition exists and
that the system should be scrutinized for abnormalities.
5.10
5.10.1
5.10.2
Protection and Automation Functions
5.10 49 Thermal Overload Protection
Reyrolle 7SR5, Transformer Protection Device, Device Manual
199
C53000-G7040-C015-1, Edition 11.2019
